1. Field of the Invention
This invention relates to a method for igniting a lean fuel mixture in a main chamber of an internal combustion engine, said internal combustion engine being fitted with a pre combustion chamber which holds a rich air-fuel mixture that is ignited for igniting the lean fuel mixture in the main chamber, whereas the pre combustion chamber is fuelled using a controlled valve.
2. The Prior Art
In internal combustion engines (ICE) the combustion must be carefully monitored and controlled in order to optimize the performance and minimize emissions. In large industrial engines a commonly used strategy to reduce the NOx emissions is to use lean fuel mixtures, i.e., fuel mixtures with excessive air as compared to stoichiometric mixtures (mixtures of fuel and air whereas the amount of air is equal to the theoretical amount needed for the complete combustion of the fuel). However, lean mixtures are difficult to ignite. Failure to ignite the fuel mixture is unacceptable because then the unburned fuel is a pollutant in itself. To overcome the problem of misfires (failure to ignite the fuel mixture) pre combustion chambers are often used which typically is a small cavity in the cylinder head or the spark plug with an opening towards the main (combustion) chamber. The pre chamber is fuelled with a rich mixture (easy to ignite) whereas the main chamber is fuelled with a lean (hard to ignite) mixture. The fuel in the pre chamber is then ignited and as the burning fuel propagates into the main chamber the lean mixture is ignited as a secondary effect, hereby securing stable over all combustion.
However, the combustion of the rich fuel mixture in the pre chamber itself may cause unacceptable emissions, specifically disproportionate amounts of NOx and CO. It is well known that an unacceptable amount of emissions come from over fuelling of pre chambers. But it is very difficult to monitor the fuel admission in the pre chamber such that it becomes rich enough to reliably be ignited but not richer than that to avoid excess emissions. Clearly, there is a trade off between stable combustion and excess emission.
In commonly used solutions the fuel admission to the pre chamber is calibrated. However, the optimal fuel amount (giving stable ignition subject to minimum emission) may change over time due to changes in turbo charge. Moreover, component wear in the fuel admission system may cause that imprecise, excessive or insufficient fuel is injected. Finally the optimum fuel amount for each individual pre chamber will vary due to unique mixing and scavenging characteristics of each engine cylinder.
Another state of the art pre chamber fuel control is done by check valves. The check valve opens when the pressure in the pre chamber falls below the fuel supply pressure. Thus the pre chamber fuelling typically starts at the beginning of the main chamber gas exchange phase and stops right after beginning of compression. This concept flashes the pre chamber and fuels the main chamber during scavenging. Partly this fuel is wasted directly through the exhaust port. Adjustment of the air fuel ratio is just possible by changing the fuel supply pressure. State of the art controls use for this purpose a functional correlation to air manifold pressure or load. Mapping of pre chamber performance is rather complicated and not practice. Hence a precise pre chamber air fuel ratio adjustment over a range of load, speed, main chamber air fuel ratio and cylinder level variation is impossible. Furthermore check valves are subject to contamination and clogging. Besides imprecise fuelling a check valve pre chamber is also characterized by a high level of inconsistency.
The U.S. Pat. No. 7,275,916 B2 shows an integrated engine/compressor control for gas transmission compressors that uses engine operating data, e.g. engine speed, intake manifold air pressure, ignition system energy, etc., and compressor operating data, e.g. load steps on each cylinder, suction pressure, discharge pressure, etc. The controller then processes this data to determine various control parameters for engine, e.g. ignition timing, pre-chamber fuelling quantity, pre-chamber fuelling rate, etc., and compressor, e.g. compressor load step, compressor suction and/or discharge bottle conditions, etc. The controller can be programmed to achieve engine optimization in terms of fuel consumptions and emissions. Such a control requires numerous sensors providing different input signals to the controller and, hence, is very costly.
It is an object of the invention to provide a system and a method for igniting a lean fuel mixture in a main chamber of an internal combustion engine, which ensures a stable and consistent ignition of the main chamber and simultaneously reduces the emission of the internal combustion engine.